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Water Infiltration into Exposed Fractured Rock Surfaces

School of Forest Resources, Univ. of Georgia, Athens, GA 30602

D. D. Evans

Dep. of Hydrology and Water Resources, Univ. of Arizona, Tucson, AZ 85721

*Corresponding author.

ABSTRACT

Fractured rock media are present at many existing and potential waste disposal sites, yet characterization data and physical relationships are not well developed for such media. This study focused on water infiltration characteristics of an exposed fractured rock as an approach for defining the upper boundary condition for unsaturated-zone water percolation and contaminant transport modeling. Two adjacent watersheds of 0.24 and 1.73 ha with slopes up to 45% were instrumented for measuring rainfall and runoff. Fracture density was measured from readily observable fracture traces on the surface. Three methods were employed to evaluate the rainfall-runoff relationship. The first method used the annual totals and indicated that only 22.5% of rainfall occurred as runoff for the 1990–1991 water year, which demonstrates a high water intake rate by the exposed fracture system. The second method employed total rainfall and runoff for individual storms in conjunction with the commonly used USDA Soil Conservation Service curve number method developed for wide ranges of soils and vegetation. Curve numbers between 75 and 85 were observed for summer and winter storms with dry antecedent runoff conditions, while values exceeded 90 for wet conditions. The third method used a mass-balance approach for four major storms, which indicated that water intake rates ranged from 2.0 to 7.3 mm h^-1, yielding fracture intake velocities ranging from 122 to 293 m h^-1. The three analyses show the complexity of the infiltration process for fractured rock. However, they contribute to a better understanding of the upper boundary condition for predicting contaminant transport through an unsaturated fractured rock medium.

Received for publication January 14, 1992.

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